1. Field of the Invention
The present invention relates to a solid state image sensor and, more particularly to, a solid state image sensor suitable for enhancing sensitivity of charge coupled devices (CCDs) using phase shift of light, and a method for fabricating the same.
2. Background of the Related Art
In general, a solid state image sensor refers to a device for image sensing an object and converting the image to electrical signals using photoelectric conversion elements and charge coupled elements.
The charge coupled elements are used to transmit signal charges generated in the photoelectric conversion elements (i.e., photodiodes) passed through a color filter layer in a specified direction through a microlens using variations of electric potential in a substrate.
The solid state image sensor includes a plurality of photodiodes (PDs), vertical CCDs each provided between the PDs for transmitting charges generated from the PDs in the vertical direction, horizontal CCDs for transmitting the charges vertically transmitted by the vertical CCD in the horizontal direction, and floating diffusion devices for sensing and amplifying the horizontally transmitted charges and outputting the amplified charges to the peripheral circuits.
A related art method for fabricating a solid state image sensor will be described with reference to the attached drawings. FIGS. 1a-1d illustrate sections showing the steps of a related art method for fabricating a solid state image sensor.
As shown in FIG. 1a, a first flat layer 14 is formed on a black-and-white solid state image sensor 11, which is provided with a plurality of PDs 12 for converting image signals of light to electrical signals, vertical CCDs 13 for transmitting image charges generated in the PDs 12 in the vertical direction, and horizontal CCDs (not shown) for transmitting the vertically transmitted image charges in the horizontal direction.
As shown in FIG. 1b, first to third color filter layers 15, 16 and 17 are selectively formed on the first flat layer 14 in succession to be in correspondence to the PDs 12 on a one-to-one basis. The color filter layers are formed by coating and patterning a colorable resist and then dying and fixing the patterned colorable resist layer.
As shown in FIG. 1c, a second flat layer 18 is formed on en entire surface including the first to third color filter layers 15, 16 and 17.
And, as shown in FIG. 1d, a plurality of microlenses 19 are formed on the second flat layer 18 to be in correspondence to the PDs 12 on a one-to-one basis.
The operation of the related art solid state image sensor as fabricated above will be described.
The light incident to the solid state image sensor is focused by the microlenses 19, passes through the first to third color filter layers 15, 16 and 17 each of which passes a light of a particular wavelength, and is incident to the PDs 12 on a one-to-one basis.
The light incident to the PDs 12 is converted to an electrical signal and transmitted in the vertical and horizontal directions according to clock signals applied to the gates on the vertical and horizontal PDs of the solid state image sensor. The electrical signals are then sensed and amplified by the floating diffusion device (not shown) at the end of the horizontal PD and transmitted to the peripheral circuits.
The above-stated related art method for fabricating a solid state image sensor, however, involves many problems as follows:
First, spherical surface control of the microlenses is difficult, and dependent on process parameters.
Second, as a contact depth of the PDs becomes greater, a depth of focus cannot be made greater in focusing.
Third, the microlenses formed of a transparent resinous material is susceptible to deformation, and has a low thermal resistance to cause irregular reflection of light.
It is, therefore, an object of the present invention to provide a solid state image sensor and its fabricating method designed to increase a depth of focus using phase shift of light and prevent irregular reflection of light, thereby enhancing focusing efficiency and hence sensitivity.
To achieve the object of the present invention, there is provided a solid state image sensor including a plurality of photodiodes for generating image charges from incident lights, a plurality of charge coupled devices provided between the photodiodes for transmitting the image charges in one direction, a first flat layer formed on an entire surface of the photodiodes and the charge coupled devices, a plurality of color filter layers formed on the first flat layer to be in correspondence to the photodiodes, a plurality of black layers formed on the first flat layer between the color filter layers, a plurality of phase shift layers selectively formed on the color filter layers to be in correspondence to the photodiodes alternately, a second flat layer formed on an entire surface including the phase shift layers, and a plurality of microlenses formed on the second flat layer to be in correspondence to the photodiodes.
In accordance with another aspect of the present invention, there is provided a method for fabricating a solid state image sensor, comprising the steps of forming a plurality of photodiodes for generating image charges from incident lights, forming a plurality of charge coupled devices between the photodiodes for transmitting the image charges in one direction, forming a first flat layer on an entire surface of the photodiodes and the charge coupled devices, selectively forming a plurality of color filter layers on the first flat layer to be in correspondence to the photodiodes, forming a plurality of black layers on the first flat layer between the color filter layers, selectively forming a plurality of phase shift layers on the color filter layers to be in correspondence to the photodiodes alternately, forming a second flat layer on an entire surface including the phase shift layers, and forming a plurality of microlenses on the second flat layer to be in correspondence to the photodiodes.